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Analytical Technique Whole Rock Geochemistry Whole-Rock Samples

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Analytical Technique Whole Rock Geochemistry Whole-Rock Samples Analytical technique Whole rock geochemistry Whole-rock samples were analyzed for major oxides and trace elements by X-ray fluorescence (XRF) spectroscopy using a Phillips PW 1400 at the Centre d’Analyses Minérale, University of Lausanne, Switzerland. Rare-earth element and trace element concentrations were also determined by inductively coupled plasma mass spectrometry (ICP-MS) using a Hewlett Packard 45000 at the Institute F.- A. Forel, Versoix, Switzerland. Neodymium isotope analyses were carried out at the Department of Mineralogy, University of Geneva, Switzerland. Sm and Nd concentrations for age-correcting the isotopic data used the values determined by ICP-MS. Repeated analyses of Sm/Nd ratios using the Geological Survey of Japan JB-3 (basalt) standard yields a precision (2 σ) of better than 2.75%. Samples for isotopic and ICPMS analysis were digested in a 5 ml of ca 40% HF and 1 ml of ca 15M HNO 3. Samples were digested in closed PTFE beakers for one week at a temperature of 170 °C. When dry, the sample were treated with 2 x 2 ml additions of ca 15M HNO 3 followed by 1 ml of ca 6M HCl. 5 ml of ca 6M HCl was then added and left to stand overnight at 170 °C. Following digestion, samples for isotopic analysis were converted to nitrate by 3 x 1 ml additions of 4M HNO 3, dissolved in 2 ml of 1M HNO 3, centrifuged and then loaded onto the ion exchange columns. Teflon-distilled HCl and HF and Ultrex-II HNO3 were used throughout the dissolution and chemical separation procedures, and reagent and total procedure blanks are neglible. Nd separation employed Eichrom TRU-Spec Resin (50 - 100 µm) in disposable PP columns in series with Ln-Spec Resin (50 - 100 µm), using the methods described in Pin & Zaldegui (1997). Neodymium isotope ratios were measured on a seven-collector Finnigan MAT 262 thermal ionization mass spectrometer with extended geometry and stigmatic focusing using double Re filaments. 143 Nd/ 144 Nd was measured in a semidynamic mode (quadruple collectors, measurement jumping mode), mass fractionation corrected to 146 Nd/ 144Nd = 0.721903, and normalized to the La Jolla standard = 0.511837. The mean of 101 replicated analyses of this standard is 0.511837 ± 2 (2 σ). U-Pb zircon geochronology Zircons were separated from several kilograms of sample by conventional means. The sub-300 µm fraction was processed using a Wilfey table, and then the Wilfey heavies were passed through a Frantz magnetic separator at 1 A. The non-paramagnetic portion was then placed in a filter funnel with di iodomethane. The resulting heavy fraction was then passed again through the Frantz magnetic separator at full current. A side slope of 1º was used to separate non- paramagnetic zircons for ion microprobe analysis to maximize concordance. All zircons were hand picked in ethanol using a binocular microscope. Ion microprobe analytical technique Zircons were mounted in a resin disk along with the zircon standard and polished to reveal the grain interiors. The mounts were gold-coated and imaged with a Hitachi S-4300 scanning electron microscope (SEM), using a cathodoluminescence probe (CL) to image internal structures, overgrowths and zonation. Secondary electron mode (SE) imaging was employed to detect fractures and inclusions within the grains. U–Th–Pb zircon analyses (Table DR4) were performed on a Cameca IMS 1270 ion-microprobe following methods described by Whitehouse and Kamber (2005) which were modified from Whitehouse et al. (1999). U/Pb ratio calibration was based on analyses of the Geostandards zircon 91500, which has an age of 1065.4±0.3 Ma and U and Pb concentrations of 80 and 15 ppm, respectively (Wiedenbeck et al., 1995). Replicate analyses of the same domain within a single zircon were used to independently assess the validity of the calibration. Data reduction employed Excel macros developed by Whitehouse at the Swedish Natural History Museum, Stockholm. Age calculations were made using Isoplot version 3.02 (Ludwig, 2003). U–Pb data are plotted as 2 σ error ellipses (Fig. 3). All age errors quoted in the text are 2 σ unless specifically stated otherwise. Presence of common Pb was monitored using the 204 Pb signal. With the exception of one analysis, none of the samples exhibited levels of 204 Pb that were significantly elevated relative to the detector background (averaged over the entire session). Therefore no common Pb corrections were applied. The one analysis (DC 10/1/5, spot 2) which exhibited significant levels of 204 Pb (f 206 = 1.01%) is interpreted as contaminated by significant amounts of present day terrestrial Pb, as it lies on a discordia (Fig. 3) between the concordia age and the modern day average terrestrial common Pb composition of 207 Pb/ 206 Pb = 0.83 (Stacey and Kramers, 1975). A detailed rationale for choosing present day Pb as a contaminant is given by Zeck and Whitehouse (1999). References DEPAOLO , D. J. 1981. Neodymium isotopes in the Colorado Front Range and crust-mantle evolution in the Proterozoic. Nature, 291 , 193-6. LUDWIG , K.R. 2003. User’s Manual for Isoplot 3.00. A Geochronological Toolkit for Microsoft Excel. Berkeley Geochronology Center. Special Publication No . 4. PIN , C. & ZALDUEGUI , J.F.S. 1997. Sequential separation of light rare-earth elements, thorium and uranium by miniaturized extraction chromatography: Application to isotopic analyses of silicate rocks. Analytica Chimica Acta, 339 , 79-89. STEIGER , R. H. & JÄGER , E. 1977. Subcommission on geochronology: Convention on the use of decay constants in geo- and cosmochronology. Earth and Planetary Science Letters, 36 , 359- 362. WHITEHOUSE , M.J., & KAMBER , B.S. 2005. Assigning dates to thin gneissic veins in high-grade metamorphic terranes: A cautionary tale from Akilia, southwest Greenland. Journal of Petrology , 46 , 291-318. WHITEHOUSE , M.J., KAMBER , B.S., & MOORBATH , S. 1999. Age significance of U–Th–Pb zircon data from Early Archaean rocks of west Greenland: a reassessment based on combined ion- microprobe and imaging studies. Chemical Geology (Isotope Geoscience Section) , 160 , 201–224. WIEDENBECK , M., ALLÉ , P., CORFU , F., GRIFFIN , W., MEIER , M., OBERLI , F., VON QUADT , A., RODDICK , J.C., & SPIEGEL , W. 1995. Three natural zircon standards for U–Th–Pb, Lu–Hf, trace element and REE Analysis. Geostandards. Newsletter , 19 , 1–23. ZECK , H.P., & WHITEHOUSE , M.J. 1999. Hercynian, Pan-African, Proterozoic and Archaean ion microprobe zircon ages for a Betic-Rif core complex, Alpine belt, W. Mediterranean consequences for its P–T–t path. Contributions to Mineralogy and Petrology , 134 , 134–149. Table 1a. XRF major and trace Sample Number SiO 2 TiO 2 Al 2O3 Fe 2O3 MnO MgO CaO Na 2OK2OP2O5 LOI DC-10/01/06 76.57 0.21 12.92 1.79 0.05 0.84 0.71 5.47 0.45 0.24 1.09 DC-08/01/26 41.29 1.04 18.79 12.62 0.18 7.94 13.07 1.40 0.04 0.44 3.28 DP-11 51.45 0.87 12.89 11.65 0.23 6.71 12.04 2.26 0.42 0.12 0.92 DC-08/01/24 64.94 1.14 16.23 7.33 0.08 2.03 0.25 1.36 3.86 0.24 2.60 DC-08/01/25 57.90 0.94 15.84 9.94 0.34 4.69 2.08 1.94 2.29 0.34 3.58 Table 1b . ICPMS data Sample Number Sc Ti V Cr Co Ni Cu Zn Rb Sr Y DC-10/01/06 5.39 651 18.7 6.33 3.15 1.24 35.0 21.8 8.1 140 11.5 DC-08/01/26 25.1 3250 258 105 34.2 35.0 32.6 61.2 1.44 243 15.0 DP-11 21.6 2726 255 54.8 28.8 25.9 29.8 53.6 13.6 357 12.5 DC-08/01/24 7.9 3506 71 53.2 21.5 22.3 8.0 52.3 105.8 42 13.0 DC-08/01/25 13.7 2893 150 157.3 35.5 73.3 60.0 76.6 71.1 114 16.5 *Normalized to the carbonaceous chrondrite values of McDonough & Sun (1995) Table 1c. XRF trace (long count time) Sample Number Nb Zr Y Sr U Rb Th Pb Ga Zn Ni DC-10/01/06 2.8 99 22 146 <2< 7.9 3 <2< 13 33 3 DC-08/01/26 1.9 30 26 234 <2< 1.2 3 <2< 18 105 78 DP-11 1.6 28 22.2 338 <2< 12.1 <2< 9 19 88 51 DC-08/01/24 18.3 319 24.4 45 <2< 106 9 5 23 81 48 DC-08/01/25 14 115 29.5 115 <2< 69.4 7 14 20 122 163 Cr 2O3 NiO Total Nb Zr Y Sr U Rb Th Pb Ga Zn 0.00 0.00 100.33 4 81 29 137 2 8 5 7 14 37 0.03 0.01 100.14 3 50 27 235 <2< 3 <2< 14 18 102 0.02 0.01 99.58 4 46 22 346 <2< 12 2 27 17 86 0.01 0.01 100.08 19 314 28 42 3 101 11 8 23 83 0.05 0.02 99.95 15 117 34 114 5 68 8 21 21 121 Zr Nb Ba La Ce Pr Nd Sm Eu Gd Tb Dy Ho 63.6 1.93 1004 5.95 12.6 1.77 7.35 1.94 0.47 2.67 0.50 3.36 0.78 39.8 0.41 54.7 1.82 5.04 0.99 5.83 2.41 1.27 3.67 0.67 4.39 1.01 29.9 0.45 255 1.56 4.36 0.84 4.96 2.01 0.83 3.21 0.58 3.82 0.88 199.4 18.46 643 44.37 81.51 9.90 37.32 7.05 1.49 6.01 0.84 4.46 0.94 76.0 13.64 494 35.20 66.74 7.87 30.61 6.40 1.67 6.51 0.97 5.56 1.17 Cr V Ce Ba La 6 23 13 876 4 194 313 <3< 32 <4< 98 313 <3< 257 <4< 97 92 68 567 42 309 208 63 456 22 Cu Ni Co Cr V Ce Nd Ba La S Hf Sc As 46 <2< 8 11 33 15 4 1053 8 131 8 20 <3< 50 79 63 214 367 20 13 45 <4< 17 9 44 3 45 44 46 100 327 18 11 256 13 33 7 38 14 12 42 30 98 117 149 56 660 49 3 11 17 8 69 149 48 330 254 87 30 532 57 180 6 47 129 Er Tm Yb Lu Hf Ta Pb Th U La/Sm La/Sm N* ΣREE 2.33 0.33 2.32 0.35 2.35 0.39 0.44 3.19 0.79 3.07 1.92 43 2.94 0.42 2.76 0.42 1.41 0.80 b/d 0.27 0.22 0.75 0.47 34 2.52 0.37 2.44 0.36 1.06 0.38 7.98 0.26 0.20 0.78 0.49 29 2.57 0.35 2.38 0.35 4.93 1.62 4.09 17.81 1.90 6.29 3.93 200 3.21 0.44 2.91 0.43 1.88 1.16 12.66 11.33 2.96 5.50 3.43 170 Table 2.
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